Driving force from an engine of an automobile is transmitted to the wheels through a power transmission system including any or all of a transmission, a propeller shaft, a differential, and a drive shaft.
In the power transmission system, a tapered roller bearing having a high load-carrying capacity for radial load and excellent shock-resistant properties is often used as a bearing that supports a shaft. As shown in FIG. 6, the tapered roller bearing generally includes an inner ring 2, an outer ring 4, a plurality of tapered rollers 5, and a cage 6. The inner ring 2 has a conical raceway 1 on an outer circumferential side. The outer ring 4 has a conical raceway 3 on an inner circumferential side. The tapered rollers 5 are disposed between the inner ring 2 and the outer ring 4 such as to roll freely. The cage 6 holds the tapered rollers 5 at a predetermined interval in a circumferential direction.
As shown in FIG. 7, the cage 6 includes a pair of ring-shaped sections 6a and 6b, and columnar sections 6c that connect the ring-shaped sections 6a and 6b. The tapered rollers 5 are housed in pockets 6d formed between columnar sections 6c that are adjacent along the circumferential direction.
In the tapered roller bearing, the tapered rollers 5, the raceway 1 of the inner ring 2, and the raceway 3 of the outer ring 4 are in linear contact. The tapered roller bearing is designed such that the raceway 1 of the inner ring 2, the raceway 3 of the outer ring 4, and a roller center axis O match at a point (not shown) on an axial center P.
Therefore, when a load is applied, the tapered rollers 5 are pressed towards a large-end side. A shoulder section 7 that projects towards an outer-diameter side is provided on a large-diameter side of the inner ring 2 to receive the load. To prevent the tapered rollers 5 from falling towards a small-end side before the bearing is assembled to a machine and the like, a projecting shoulder section 8 is also provided on the small-end side of the inner ring 2.
As advances are made, such as downsizing the engine room to increase interior space of the vehicle, increasing output from the engine, and increasing stages of the transmission to improve fuel efficiency, the usage environment in which the tapered roller bearing is used is becoming increasingly challenging by the year. To fulfill the life span of the bearing in the usage environment, the life of the bearing is required to be further extended.
Against such a backdrop, a proposal has been made in which the load-carrying capacity is increased from a current load-carrying capacity without changes being made to the dimensions, by the number of rollers being increased or the length of the rollers being increased. As a result, the life of the bearing is extended. However, in a current structure, the shoulder section (small shoulder) 8 is provided on the inner ring on the small-diameter side of the raceway, due to reasons regarding bearing assembly, as described above. Therefore, the shoulder section 8 limits the increase in the length of the tapered rollers 5. Moreover, each tapered roller 5 is held by the cage 6 as described above, and the columnar sections 6c of the cage 56 are interposed between tapered rollers 5 that are adjacent in the circumferential direction. Therefore, the columnar sections 6c limit the increase in the number of tapered rollers 5. In this way, conventionally, the increase in load-carrying capacity is limited.
Conventionally, a bearing is known in which the shoulder section (small shoulder) on the small-diameter side of the inner ring is omitted (Patent Document 1). When the shoulder section on the small-diameter side of the inner ring is omitted, the length of the tapered rollers in an axial direction can be extended by an amount corresponding to the omission. The load-carrying capacity can be increased.
Patent Document 1: Japanese Patent Laid-Open Publication No. 2002-54638